Magnetic inductive flow meters use Faraday's law of induction to determine the flow velocity of a through-flowing fluid. A magnetic field is generated perpendicular to the direction of flow. In this magnetic field, charges which are transported with the fluid generate a voltage perpendicular to the magnetic field and to the flow direction, which voltage can be tapped with the aid of electrodes. The measurement voltage thus determined is proportional to a flow velocity determined over the flow cross-section. Flow meters of this type must, in order to ensure measurement accuracy, maintain largely constant geometric dimensions, irrespective of the pressure of the through-flowing fluid prevailing at any time. This pressure resistance is frequently achieved by a steel measuring tube through which the fluid flows. On the other hand, this measuring tube must not interfere with the electric and magnetic fields which permeate the fluid in the region of a measuring section. For this reason, linings or inserts are used in the measuring tube which are typically produced from ceramic or plastic materials. These fulfill the requirements of being electrically non-conductive and of scarcely affecting the magnetic field. At the same time, they protect the metal wall of the measuring tube against corrosion. Here, inserts made of a plastic material have the advantage that they are particularly easy to handle. Especially in the case of hardened plastic linings or inserts, there is, however, the problem that they are either not sufficiently dimensionally stable or that they lose their dimensional stability over time. For example, a plastic insert tends, where a pressure that is substantially lower than atmospheric pressure occurs, to detach itself from the inner wall of the measuring tube, thereby reducing the flow cross-section.
From EP 0 895 066 A1, a tubular insert for a magnetic inductive flow meter is known, which insert is composed of an elastomer, preferably synthetic rubber, as an electrically insulating material. In order for the tubular insert to be less elastic in its measuring section, strips made of steel were embedded in the rubber in this area. In the area of the two end sections, no reinforcement is present. No details are given in this printed publication about how the steel strips are embedded in the rubber, i.e. how a reinforced tubular insert can be produced. The known insert has the disadvantage that the rubber on the inner surface of the steel strips is not permanently fastened and the geometric dimensions of the tubular insert can consequently change in the event of pressure fluctuations.
From U.S. Pat. No. 5,773,723 a magnetic inductive flow meter comprising a measuring tube is known, the inner surface of which is lined with perfluoralkoxy (PFA). This lining is stabilized in a measuring section by an embedded metal grid so that the internal diameter of the tube remains constant. The essentially tubular metal grid is formed by spiral bending of a grid strip and welding of the abutting edges of the grid strip. Multiple wires running parallel to the tube axis and distributed evenly over the circumference of the tube are welded onto the outer surface of the grid. These serve to ensure a constant spacing between the inner wall of the measuring tube and the outer surface of the grid. The grid is inserted into the measuring tube and centered there by the welded wires. On the two end sides, the tubular metal grid is welded to the inner surface of the measuring tube. The lining material made of PFA is inserted between an injection mould inserted in the measuring tube and the inner wall of the measuring tube in an injection molding method, flows through the openings in the metal grid and fills the interspace between the grid and the inner wall of the measuring tube, forming a layer of constant thickness. The known lining of a measuring tube has the disadvantage, however, that its production is comparatively costly. Moreover, it is difficult when producing the tubular metal grid from a spirally bent metal grid strip to mould on tube ends running conically. Conically running tube ends of the inner cross-section are, however, frequently provided in magnetic induction flow meters in order to achieve a higher flow velocity and thus a better measuring accuracy in the measuring section.